I didn't read all of the posts on this thread- just saw you mentioned the float test. I just thought I'd add here, we did the float test for 10 eggs in our incubator who hadn't hatched by day 25. The float test said about 7 of them definitely should have been viable... we opened the eggs in a last desperate attempt to help. They were all dead- the float test did not work for us.
I hate the float test! Well, correction, it's good for eggs you want to eat, to determine if they are fresh or old.

It definitely cannot tell you that a chick is dead. No way. Sinkers are not always dead. Floaters are not always alive. It *can* let you know one is alive, if it moves, but if it doesn't move, that does not mean its absolutely dead. And if the chick is alive, a good candling should reveal it anyway, making float test irrelevant.